ITU-T defines OTN as a set of Optical Network Elements connected by optical fiber links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals. ITU Standard G.709 is commonly called Optical Transport Network (OTN) or digital wrapper technology. OTN is currently offered in three rates, OTU1, OTU2, and OTU3, and future rates are expected such as OTU4, where OTU stands for Optical Channel Transport Unit. OTU provides the electrical functions to support the management of an optical channel section, i.e., section monitor (section trail trace identifier, section error detection code (BIP-8), defect indication functions, general communications channel), and provides for transport of the optical channel data unit (ODUk). OTU1 has a line rate of approximately 2.7 Gb/s and was designed to transport a SONET OC-48 or an SDH STM-16 signal. OTU2 has a line rate of approximately 10.7 Gb/s and was designed to transport an OC-192, STM-64 or 10 Gbit/s WAN. OTU2 can be overclocked (non-standard) to carry signals faster than STM-64/OC-192 (9.953 Gb/s) like 10 gigabit Ethernet LAN PHY coming from IP/Ethernet switches and routers at a full line rate (approximately 10.3 Gb/s). This is specified in G.Sup43 and called OTU2e. OTU3 has line rate of approximately 43 Gb/s and was designed to transport an OC-768 or STM-256 signal. OTU4 is currently under development to transport future 100 GbE signal. The ODUk provides the electrical functions to support the management of an optical channel data path, i.e., path monitor (path trail trace identifier, path error detection code (BIP-8), defect indication functions, general communications channel, automatic protection switching channel), provides the electrical functions to support the management of tandem optical channel data paths, i.e., tandem connection monitors (tandem connection trail trace identifier, tandem connection path error detection code (BIP-8), defect indication functions, automatic protection switching channel), and provides for transport of the optical channel payload (OPUk).
Of note, OTN is defined in various standards such as: ITU-T G.709 Interfaces for the optical transport network (OTN); ITU-T G.798 Characteristics of optical transport network hierarchy equipment functional blocks; OTN Standard FEC (Called GFEC sometimes) is defined in ITU-T G.975; OTN Jitter is defined in ITU-T G.8251: The control of jitter and wander within the optical transport network (OTN); G.870: Terms and definitions for Optical Transport Networks (OTN); G.871: Framework for optical transport network Recommendations; G.873.1: Optical Transport Network (OTN): Linear protection; G.874: Management aspects of the optical transport network element; G.874.1: Optical transport network (OTN): Protocol-neutral management information model for the network element view; G.959.1: Optical transport network physical layer interfaces; G.8201: Error performance parameters and objectives for multi-operator international paths within the Optical Transport Network (OTN). In addition to the OTN Standard FEC (GFEC), a proprietary FEC could be used.
OTN can be utilized with a signaling and routing protocol to provide automatic resource discovery, distributing network resource information, establishing and restoring connections dynamically across the network, and the like. Exemplary signaling and routing protocols include Optical Signaling and Routing Protocol (OSRP), Automatically Switched Optical Networks (ASON), Generalized Multi-Protocol Label Switching (GMPLS), and the like. The signaling and routing protocol can be utilized to provide OTN mesh networks. In OTN mesh networks, unused or protect OTUk/ODUk lines are usually transmitting the open channel signal (OCI) or other maintenance signal in the ODUk overhead. The signaling and routing protocol operates on a control plane in a network and provides automatic resource discovery, distribution of network resource information, establishment and restoration of connections dynamically across the network, and the like. For example, OSRP is a distributed protocol designed for controlling a network of optical cross-connects (OXCs). OSRP introduces intelligence in the control plane of an optical transport system. It can perform many functions such as automatic resource discovery, distributing network resource information, establishing and restoring connections dynamically across the network, and the like.
Referring to FIG. 1, a diagram illustrates the OTUk frame structure 100 mapping an ODUk 102 into an OTUk 104 according to OTN. In-band OSRP or GMPLS messages are transmitted and received using the General Communication Channel (GCC) in the OTUk/ODUk overhead; these bytes have a specified location relative to the OTUk frame. Unlike the SONET/SDH line/MS and path relationship, there is no timing adaptation between the OTUk 104 and the ODUk 102. The OTUk 104 frame is an extension of the ODUk 102 frame. The relative location of the ODUk 102 and OTUk 104 overhead is fixed, and the mapping of ODUk/OTUk is synchronous—meaning that the reference clock for the OTUk 104 information must also be the reference clock for the ODUk 102 information.
Referring to FIG. 2, an exemplary OTN network 200 includes four nodes 202a-202d with OTUk lines between each node 202a-202d. The OTUk lines are designated OTUk AB, OTUk BD, OTUk CD, and OTUk AC to designate the two endpoints of each line. The OTN network 200 is configured to utilized OSRP running over the in-band GCC on each OTUk line. A single ODUk sub-network connection (SNC) 204 is routed from node 202a through node 202b to node 202d at corresponding Trail Termination Points TTP A1, TTP B1, TTP B2, and TTP D1 {A1-B1-B2-D1}. The corresponding OTUk interfaces that are not supporting any cross-connects (i.e., TTPs A2, C1, C2, D2) transmit OCI 206. Timing for these interfaces (i.e., TTPs A2, C1, C2, D2) comes either from a local source on the node, or from an external network reference. In the event of a failure along the interfaces A1-B1-B2-D1, setup and connect messages are signaled back and forth between the nodes 202a-202d to reserve bandwidth and set up cross-connects. The circuit is re-routed to A2-C1-C2-D2. While the new ODUk circuit is being provisioned, each OTUk line must re-frame on the ODUk that originates on node 202a. During this re-framing, GCC signaling messages are dropped.
The above scenario can be extended to the case where an OTUk line is advertising mixed capability (e.g., ODU2, ODU1, ODU0, OPVC1), and the system has to configure a multiplex structure to support the requested circuit. In these cases the OTUk also loses frame momentarily and has the potential to drop GCC messages. RSVP protocols can retransmit dropped frames, but retransmission relies on timers in the protocol, and network restoration performance can be seriously affected. For example, OSRP has a four second timer for Setup and Connect retransmission. Carrier-grade transport networks often require restoration times below 100 ms.
Conventionally, there are a couple of solutions to overcome the dropped GCC messages and retransmission to speed up network restoration times. For example, an out-of-band data network could be used to run the routing and signaling protocols. Disadvantageously, this is more costly and requires an overlaid network, and can have performance issues if the network is shared with other applications. Higher order ODUk terminations could be pre-provisioned, i.e. without dynamic restoration through the signal and routing protocol. Here, the network 200 could only support routing of lower order ODUj signals. Disadvantageously, this severely limits the network, but maintains a communication channel through the pre-provisioned OTUk/ODUk GCC. Finally, conventional solutions simply rely on Setup/Connect re-transmission when messages are dropped accepting the resulting mesh restoration times above carrier-grade requirements. None of these solutions is acceptable in an ODUk transport network where fast mesh restoration is required.